The invention relates to an apparatus and method for processing a received digital signal such that the resultant signal has a very high probability of matching, bit-by-bit, the originally transmitted signal.
Digital signals carry information comprised of binary digits of data, known simply as bits. The bits have a logic state characterized either as zero or one, and a change in logic state is a bit transition. Multiple bits may be grouped to represent characters, such as letters, numbers or control signals. Multiple characters may be grouped in packets to form messages. The packets may be transmitted via a radio frequency ("RF") carrier for receipt at a remote location as is well known in the prior art. Packets may also be transmitted along optical fibers and other known communications media.
When transmitting digital information from point-to-point it is usually desired that the received signal precisely match the transmitted signal, i.e., that the radio transmission be received with a very low error rate. A low error rate is important for many applications such as in a digital encryption system used for communication security. This can be accomplished when the data in the transmitted signal and the data in the received signal are identical, or nearly so. Problems arise, however, when the transmitter and receiver are remote and the mode of transmission is means such as high-frequency (HF) radio or via satellite. Atmospheric interference, topographic interference, and multipath time delays caused by travel over different transmission paths often distort the received signal, increasing the error rate.
If an error-free transmission path cannot be assured, the signal may be restored to its original form at the receiving site. One known method is to use multiple receivers that are either diverse in space or diverse in frequency (if the transmitter is similarly configured) and to accept the signal from the receiver adjudged to have the best (or most accurate) signal. One problem often encountered in such systems is the framing or aligning of bits in time from several receivers. While all receivers are receiving the same source signal, that signal has been affected by the different interferences encountered in the different paths taken to reach the plural receivers, by the differences in the response of the receivers, and by the different lengths of the paths to geographically diverse receivers.
The Ikeda and Mitani U.S. Pat. No. 4,015,205 dated March, 1977 discloses a device that elongates the bit information, causing the bits from plural received signals to overlap, assuring an uninterrupted data flow when receivers are switched in response to changing signal strength. Time aligning is accomplished in the Shiki and Ohmori U.S. Pat. No. 4,384,358 dated May 17, 1983 with heterodyne receiver circuits and a phase detector to zero the phase difference.
Another complex problem often encountered is selecting which signal is the most accurate. Two known techniques for signal selection are: (1) selecting the signal with the best signal-to-noise ratio (including variations such as selecting the strongest signal) and (2) generating a new signal using information from several signals (e.g., adding portions from each received signal).
The Hamada U.S. Pat. No. 4,403,343 dated Sept. 6, 1983 discloses a switching device that selects the signal with the strongest reception and switches the output signal to the selected signal. The device disclosed in the Mohr and Stacer U.S. Pat. No. 3,651,406 dated Mar. 21, 1972 includes a switch that selects the signal with the best signal-to-noise ratio. The device in the Jayant U.S. Pat. No. 3,997,844 dated Dec. 14, 1976 selects the signal with the largest autocorrelation function value. These and similar prior art switching devices, in general, often introduce inaccuracies during the time that switching actually takes place, (e.g. one or two bits can be lost). Further, they frequently rely on the inputs from only one signal at a time and ignore the information contained in the other signals.
The receiver in the Shiki '358 patent combines plural signals after reducing their amplitude dispersion by suppressing the received signal with the greater amplitude versus frequency dispersion. Other receivers that combine signals include the Alter U.S. Pat. No. 4,347,627 dated Aug. 31, 1982, the Tatsuzawa et al. U.S. Pat. No. 4,216,428 dated Aug. 5, 1980, and the Hill U.S. Pat. No. 3,934,204 dated Jan. 20, 1976. Frequently, receivers which combine signals are not highly accurate, because invalid signals are added to valid ones and because there is often no way to assess the best output logic state when corresponding bits from received signals cancel each other (e.g., a one and a zero).
Therefore, an object of the present invention is to provide a novel and accurate signal processing apparatus and method that obviate these and other problems of the prior art.
It is another object of the present invention to provide a signal processing apparatus and method which ignores known erroneous signals.
It is yet another object of the present invention to provide a signal processing apparatus and method which uses all signals that are not known to be erroneous.
It is a further object of the present invention to provide a signal processing apparatus and method which resolves situations wherein there are equal numbers of signals with conflicting information.
It is another object of the present invention to provide a signal processing apparatus and method which avoids data losses that occur during conventional receiver switching.
Accordingly, the apparatus and method for processing a common digital signal received at plural receivers in the present invention provide a single output signal by using acceptable portions of the received signals in a voting scheme that determines the logic state of each bit in the output signal. Before the signals participate in the vote, they are synchronized in a novel multi-matrix alignment process. A voting algorithm determines the majority logic state of each frame in the received signals. The present invention also maintains statistics to determine the history of correctly receiving data from a particular signal. If the vote is a tie, the output signal uses the logic state of the corresponding bit in the signal with the best history of having correct data, or it revotes without the signal with the poorest history of being correct.